1. Field of the Invention
The present invention relates to a printhead for printing data on a printing medium by discharging ink in accordance with an inkjet printing method and, more particularly, to an inkjet printhead characterized by the layout of circuit blocks on the semiconductor substrate of the printhead having a plurality of electrothermal converters.
2. Description of the Related Art
A printhead mounted on a printing apparatus according to a conventional inkjet method has a circuit arrangement like the one shown in FIG. 4. The electrothermal converter (heater) of this printhead and its driving circuit are formed on a single substrate 400 by a semiconductor process technique, as disclosed in, e.g., Japanese Patent Laid-Open No. 5-185594. In FIG. 4, reference numerals 401 denote electrothermal converters (heaters) for generating heat energy; 402, power transistors each for supplying a desired current to a corresponding heater 401; 404, a shift register for supplying a current to each heater 401 and temporarily storing image data representing whether ink is to be discharged from the nozzle of the printhead; 405, a transfer clock (CLK) input terminal formed in the shift register 404; 406, an image data input terminal for inputting serial image data for turning on/off the heaters 401; 403, a latch circuit for latching image data for each heater in units of blocks; 407, a latch signal input terminal for inputting a latch timing signal (LT) to the latch circuit 403; 408, a block selection circuit (3-input 8-output decoder) for selecting a block; 409, 410, and 411, block selection circuit input logic signals, among which the signals 409 and 411 respectively correspond to the most and least significant bits; 419, a circuit for receiving a block selection output signal 416 and latch output signal 417 and outputting an AND; 413, a switch for determining a timing for flowing a current through the heater 401; 415, a heat signal input terminal (HEAT) for inputting a timing for controlling the switch 413; 414, a power supply line for applying a predetermined voltage and supplying a current to the heater; 412, a GND line which receives a current via the heater 401 and power transistor 402; 420, a circuit unit around the heater that includes the components 401, 402, 412, 414, 413, and 419; and 430, a unit which includes all the circuits 403, 404, 408, and 420 necessary for controlling discharge of one type of ink.
FIG. 7 shows an example of input and output waveforms to and from the block selection circuit 408 that is shown in FIG. 4. Reference numerals 409, 410, and 411 denote block selection input signals; 700 to 707, block selection output signals; and 710, a virtual timing signal which explains the timing, and takes values up to 7 such that it takes 0 for a period during which the block selection output signal 700 is at "Hi" level, and 1 for a period during which the block selection output signal 701 is at "Hi" level.
Operation of the shift register circuit 404 and latch circuit 403 that are shown in FIG. 4 will be described with reference to FIG. 8. FIG. 8 shows timings when the timing signal 710 has a value of 0. Also when the timing signal 710 has one of values of 1 to 7, signals are input at similar timings. Reference numeral 405 denotes a shift register transfer clock (CLK) signal; and 406, an image data input signal. The transfer clock input terminal 405 receives transfer clocks (CLK) by the number of bits of one block of image data stored in the shift register 404. Data is transferred to the shift register 404 in synchronism with the rise timing of the transfer clock (CLK). Image data (DATA) for turning on/off each heater 401 is input from the image data input terminal 406.
Image data stored in the shift register 404 will be called one image block. In this case, the number of heaters for one image block is eight, but can be arbitrarily set in practice. Transfer clock (CLK) pulses equal in number to the heaters 401 for one image block are input to transfer image data (DATA) to the shift register 404. Then, a latch signal (LT) is input to the latch signal input terminal 407 to latch image data corresponding to each heater in the latch circuit 403.
Referring back to FIG. 4, operation will be described again. Anyone of eight outputs 417 of the latch circuit 403 and anyone of eight outputs 416 of the decoder 408 are input to the AND circuit 419. When both the two inputs to the AND circuit 419 are at "Hi" level, a "Hi" signal is input to the switch 413. While the heat signal (HEAT) 415 for controlling the switch 413 is at "Hi" level, the switch 413 is kept on. By keeping the switch 413 on for a proper length of time by supplying the heat signal (HEAT) 415, a current flows into the power transistor 402 and heater 401 via the power supply line 414 during the ON period of the switch 413, and flows into the GND line 412. At this time, the heater 401 generates heat necessary for discharging ink, and ink corresponding to image data is discharged from the nozzle of the printhead.
The number of heaters which can be independently controlled by the latch circuit and decoder is determined by the product of the numbers of outputs 416 and 417, and in this case, 8.times.8=64 at maximum.
Reference numeral 502 denotes an ink supply hole formed at almost the center of the chip by anisotropy etching or sandblasting in order to supply ink from the rear surface of the semiconductor substrate. Inks that are supplied from the ink supply holes are supplied separately to the heaters 401 that are formed on the substrate 400 through ink passages(not shown). In accordance with the drive of the heaters, inks are supplied from orifices which are formed in the position corresponding to each heaters.
The unit 430 includes circuits necessary for discharging one type of ink that is supplied from the one ink supply hole. In FIG. 4, the circuit blocks 420 are laid out on the two sides of the ink supply hole 502. In this case, a total of 64 heaters are laid out on the two sides of the ink supply hole 502. The block selection circuit (decoder) 408, and the latch circuit 403 and shift register 404 are laid out on opposite sides via the transistor section 420. If they are laid out on the same side of the transistor section 420, the latch circuit 403, shift register 404, and decoder 408 have many elements, and the large area is required for arranging the units. Further, the latch circuit output line and decoder output line cross each other, which reduces the area and degrades reliability. Still further, the input terminals 405, 406, 407, 409, 410, and 411 must be arranged in a region on the same side of the substrate, which requires a large substrate size. For these reasons, the decoder 408, latch circuit 403, and shift register 404 are generally laid out as shown in FIG. 4.
FIG. 3 is a perspective view of the inkjet printhead which has circuit arrangement explained in FIG. 4 taken along the plane ABCD for descriptive convenience. The flow of ink will be explained with reference to FIG. 3.
An orifice plate 300 is mounted on the surface of the substrate 400, and a space for flowing ink, i.e., an ink passage 301 on the heater is defined in the orifice plate 300. An ink tank (not shown) is mounted on the lower surface of the semiconductor substrate 400, and from the lower surface side, the ink is supplied to the ink passage through the ink supply hole. Ink is guided onto each heater 401 via the ink passage 301. A current is flowed through the heater to apply heat to the ink, and ink droplets are discharged from an orifice 302, which is formed in the position corresponding to each heaters, in a direction perpendicular to the substrate plane by bubbles produced by boiling ink. Ink droplets 303 attach to a printing medium (not shown) such as a paper sheet placed parallel to the substrate plane to print data.
FIG. 5A is a block diagram showing the whole layout of a circuit that is not publicly disclosed, FIG. 5B is a block diagram showing a part A in FIG. 5A in detail, and FIG. 5C is a block diagram showing a part B in FIG. 5A in detail (to be simply referred to as "FIGS. 5A, 5B, and 5C").
The arrangement in FIGS. 5A, 5B, and 5C is obtained by simply laying out two arrangements in FIG. 4 side by side into one chip.
Reference numeral 501 denotes an ink supply hole for guiding, from the lower surface, ink different from ink of the ink supply hole 502; and 431, a unit which includes all circuits necessary for controlling discharge of one type of ink electrically similar to the block 430, and can control discharge/non-discharge of ink different from ink of the block 430 independently of the block 430.
The arrangement of FIGS. 5A, 5B, and 5C can determine the relative positions of heaters for controlling discharge of the ink of two colors by photolithography precision of the semiconductor process, and can attain a higher alignment precision than the case of laying out two arrangements in FIG. 4 side by side. A region near the cut surface must be set as an ineffective region where no element is formed because even slight chippings are generated upon cutting off a chip from a wafer. Thus, the arrangement in FIGS. 5A, 5B, and 5C requires only a smaller area than that twice the arrangement of the each unit in FIG. 4.
FIG. 6A is a block diagram showing the whole layout of a circuit that is not publicly disclosed, FIG. 6B is a block diagram showing a part A in FIG. 6A in detail, and FIG. 6C is a block diagram showing a part B in FIG. 6A in detail (to be simply referred to as "FIGS. 6A, 6B, and 6C").
The arrangement shown in FIGS. 6A, 6B, and 6C is obtained by adding functional units 601, 602, 603, 604, 605, and 606, other than a circuit for selecting a heater, to the arrangement shown in FIGS. 5A, 5B, and 5C. The circuit units 601, 602, and 603 are laid out by dividing one functional unit into three parts under limitations on a free layout region. Similarly, the circuit units 604, 605, and 606 are laid out by dividing one functional unit into three parts. An example of the functional unit is a circuit for correcting variations in heater resistance caused by variations in heater manufacturing process, e.g., variations in the film quality and film thickness of a heater material, photolithography, and etching.
The heater resistance variation correction circuit in FIGS. 6A, 6B, and 6C is constituted by a circuit 801 for detecting a heater value, a memory 802 storing the detection result, and a correction circuit 803 for making energy applied to ink uniform by changing the time for flowing a current through the heater in accordance with the value read out from the memory. The circuit 801 is formed from the same conductive film as the heater, and when the resistance of the heater varies, the resistance of the circuit 801 also varies. A voltage at the detection terminal 811 is externally read out by flowing a current through a detection terminal 811 connected to the circuit 801. The calculation result of variations in heater resistance from the voltage of the detection terminal 811 is written in the memory 802 via a data terminal 812 and write permission terminal 813. The memory 802 is a nonvolatile memory which holds the memory contents even after the power supply is turned off. The multiplexer 803 selects arbitrary one of signals 815, 816, 817, and 818 on the basis of the contents of memory outputs 820, 821, 822, and 823, and controls the switch 413 by a selected heat signal (HEAT) 830 or 831. The signals 815, 816, 817, and 818 have different pulse widths. For example, a heat signal having a small pulse width is selected for a heater having a low heater resistance because this heater flows a large current and generates a larger power in a unit time than the remaining heaters. To the contrary, a heat signal having a large pulse width is selected for a heater having a high heater resistance. In this way, the heat generation amount of the heater is controlled to be uniform in any manufacturing lot.
In the prior art, however, when a signal processing circuit other than the heater selection circuit for selecting any electrothermal converter is to be arranged, the signal processing circuit unit like an adding functional unit must be divisionally laid out into several parts because the layout region is distributed. Wiring for connecting divided units must be prolonged to transfer a signal. Compared to one unit without any division, the wiring region, substrate size, and cost increase are problems encountered.
The inkjet printer must handle a large current through the heater 401 in order to realize reliable ink discharge. This current flows into the GND 412, generating a large GND line noise. To present this, a circuit arrangement resistant to noise is demanded. When the signal processing circuit unit is divisionally laid out into several parts, a critical signal line such as an analog signal line or high-speed digital signal line susceptible to disturbance must be bypassed over the unit in some cases. If switching noise is induced from the power supply line 414 or GND 412 to a signal line susceptible to disturbance, the circuits may malfunction, which is another problem.
If the signal processing circuit unit includes an analog circuit, the temperature difference between elements within the unit, and characteristic of the element by the influence of the production process variations are increased by dividing the signal processing circuit unit into several parts. Different element operating points requiring relative precision cause a negligible offset in terms of characteristics, and the characteristics of the signal processing circuit unit degrade, which is another problem.